Typically the parameters I am looking for are, no "early extension" prior to A5. That will change the your inclination to the ground, but in "moderation". Some early extension can show that the player is using the ground properly and a bit more dynamic motion.

For one thing, it's a power piece. It has its downsides, but it can and often does add speed.

Yeah what these guys said. It's all about what's most beneficial for the player. If early extending a little helps the player swing a little faster and is more biased towards creating an outward path, then it's all good.

Yes and just to clarify, I strongly believe some early extension is a good thing. It's very rare to see a good player, including tour players, keep their heads on the "wall" for the entire downswing. Can actually take away from the dynamics. That video of me with the chair against my tailbone is almost 3 1/2 years old so I've "evolved" a little since then.

I think the knee/stick drill can be very useful for those players that excessively over flex that trail leg.

Thanks for clearing that up Mac - IMHO what happens at the head is just a culmination of many other things earlier in the chain. So early extension to me is classified at the center of the body.

In the case of the head I do believe some movement is both natural and acceptable. You only get 1 spine - consciously locking things at the neck is an awfully risky proposition when things are moving at great force underneath.

festivus...could you please briefly clarify more on hip mobility ...what is toe-touch...squat?

Sorry - sure. Toe touch is touching your toes with straight legs. Just a sign of how your body accepts and deals with flexing the trunk in general. It's not the be all end all but easy to relate to over the internet.

By squat I'm talking about just a body-weight squat, thighs should pass below parallel to the floor with the feet flat. This tells you how the abdominals, hips and ankles function together. You needed it to learn to walk...you can add another challenge by holding a golf club overhead. Yes the lifter is wearing platform shoes but note the incredible difference in hip, ankle, shoulder movement and spinal stability between these two:

One person is far more ready to keep posture under the increasing pull of centrifugal force in the golf swing.